Anticalins

ABSTRACT

The invention relates to the production of novel proteins exhibiting bonding activity for certain ligands, the so-called anticalins. To this end, the structure of peptides of the lipocalin family is modified by amino acid replacement in their natural ligand binding pocket using generic engineering methods. Alike immunoglobulin, the anticalin thus obtained can be used to identify or bond molecular structures.

The present invention relates to new polypeptides, methods for theirproduction as well as for their use for binding or for recognizing givenligands.

The lipocalins (Pervaiz and Brew, FASEB J. 1 (1987), 209-214) are afamily of small, often monomeric secretory proteins which have beenisolated from various organisms, and whose physiological role lies inthe storage or in the transport of different ligands as well as in morecomplex biological functions (Flower, Biochem. J. 318 (1996), 1-14). Thelipocalins bear relatively little mutual sequence similarity and theirbelonging to the same protein structural family was first eluicidated byX-ray structure analysis (Sawyer et al., Nature 327 (1987), 659).

The first lipocalin of known spatial structure was the retinol-bindingprotein, Rbp, which effects the transport of water-insoluble vitamin Ain blood serum (Newcomer et al., EMBO J. 3 (1984), 1451-1454). Shortlythereafter the tertiary structure of the bilin-binding protein, Bbp,from the butterfly Pieris brassicae was determined (Huber et al., J.Mol. Biol. 195 (1987), 423-434). The essential structural features ofthis class of proteins can be illustrated with the help of the spatialstructure of this lipocalin, which is schematically reproduced in FIG.1A. The central element in the folding architecture of the lipocalins isthe cylindrical β-pleated sheet structure, the so-called β-barrel, whichis made up of eight nearly circularly arranged antiparallel β-strands.

This supersecondary structural element can also be viewed as a“sandwich”-arrangement of two four-stranded β-sheet structures.Additional structural elements are an extended segment at theamino-terminus of the polypeptide chain and an α-helix close to thecarboxy-terminus, which itself is followed by an extended segment. Theseadditional features are, however, not necessarily revealed in alllipocalins. For example a significant part of the N-terminal segment ismissing in the epididymal retinoic acid-binding protein (Newcomer,Structure 1 (1993), 7-18). Additional peculiar structural element arealso known, such as for example membrane anchors (Bishop und Weiner,Trends Biochem. Sci. 21 (1996), 127) which are only present in certainlipocalins.

The β-barrel is closed on one end by dense amino acid packing as well asby loop segments. On the other end the β-barrel forms a binding pocketin which the respective ligand of the lipocalin is complexed. The eightneighbouring antiparallel β-strands there are connected in a respectivepairwise fashion by hairpin bends in the polypeptide chain which,together with the adjacent amino acids which are still partially locatedin the region of the cylindrical β-pleated sheet structure, each form aloop element. The binding pocket for the ligands is formed by these intotal four peptide loops. In the case of Bbp, biliverdin IXγ iscomplexed in this binding pocket. Another typical ligand for lipocalinsis vitamin A in the case of Rbp as well as β-lactoglobulin (Papiz etal., Nature 324 (1986), 383-385).

Alignments of the sequences from different representatives of thelipocalin family can be found in, among other publications, thepublication by Cowan et al. (Proteins: Struct., Funct., Genet. 8 (1990),44-61) and in the review article by Flower (FEBS Lett. 354 (1994),7-11). Among the currently many more than 20 different known lipocalins,there exist mainly two human proteins which have already beenbiochemically characterized in detail: the retinol-binding protein andthe apolipoprotein D, ApoD (Yang et al., Biochemistry 33 (1994),12451-12455). ApoD is especially interesting since it bears a closestructural relationship with the Bbp mentioned above (Peitsch undBoguski, New Biologist 2 (1990), 197-206).

A classical example for proteins which selectively bind ligands by wayof non-covalent interaction is represented by antibodies, i.e.immunoglobulins. These proteins play a crucial role as reagents in theareas of biotechnology, medicine, bioanalytics as well as in thebiological sciences in general. Despite the variety of the givenapplication possibilities in connection with the recognition, binding orseparation of ligands, almost exclusively immunoglobulins are currentlyused for corresponding purposes. In contrast, the application of otherproteins with defined ligand-binding characteristics, for example thelectins, has remained restricted to special cases.

Specific antibodies can be specifically produced against diverse host oftarget structures, so-called haptens or antigens. In addition to themethod for producing monoclonal antibodies, which is in the meanwhilegenerally established, biosynthetic methods have also been more recentlyemployed for this purpose, for example by using the “phage display”technique (Hoess, Curr. Opin. Struct. Biol. 3 (1993), 572-579; Wells andLowman, Curr. Opin. Struct. Biol. 2 (1992), 597-604). Once the geneticinformation for the binding region (variable domains VH and VL) of animmunoglobulin with the desired hapten or antigen specificity is known,the person skilled in the art has at his disposal many geneticengineering techniques using eucaryotic or bacterial expression systemsfor the production of this antibody, its fragments or hybrid proteinsderived from it. Yet the practical application of this class of proteinsis still sometimes plagued with disadvantages.

It is for example desirable in medical applications, such as for exampletumor imaging or drug targeting (Chester und Hawkins, Trends Biotechnol.13 (1995) 294-300) to utilize binding domains as small as possible,since with this one expects improved tissue penetration. According tothe general view, the Fv-fragment, composed of the variable domain ofthe light polypeptide chain (VL) and the variable domain of the heavypolypeptide chain (VH) of an antibody, is normally the smallestimmunoglobulin fragment which forms a structurally intactantigen-binding site. However, an Fv-fragment typically consists ofapproximately 240 amino acids, so that such a protein still exhibitsrelatively large molecular dimensions. Furthermore antibody constructionfrom two different polypeptide chains (light and heavy chain) can leadto undesirable effects. Since a respective pair of coding regions haveto be cloned and possibly expressed, recombinant production and handlingis complicated in comparison to proteins composed of a singlepolypeptide chain. In addition, experience has shown that Fv-fragmentsoften possess little proteinchemical stability, since their VL- andVH-domains are bound to each other by non-covalent interactions only.Different strategies have therefore been attempted to stabilize theassociation of both of the variable domains in the heterodimericFv-fragment. One of these methods makes use of the linking of both ofthe polypeptide chains at the translational level, wherein so-calledscFv-fragments are obtained (Bird und Walker, Trends Biotechnol. 9(1991), 132-137). However it became apparent that this proceduresometimes leads to other disadvantages, such as for example reductionsin the affinity for ligands or an undesired oligomerization behavior(Desplancq et al., Protein Eng. 7 (1994), 1027-1033).

The invention is therefore based on the goal of developing otherpolypeptide reagents which, like antibodies, exhibit specific bindingcharacteristics for given ligands. According to the invention, this taskis met with the anticalins, which are producible starting frompolypeptides of the lipocalin family by mutating amino acids which arelocated in the region of the four peptide loops at the end of thecylindrical β-pleated sheet structure, and which are characterized inthat they bind given ligands with a determinable affinity.

A topographical comparison of the course of the polypeptide chain in theprotein fold of the lipocalins with the Fv-fragments of theimmunoglobulins is shown in FIG. 2. The antigen-binding site in theimmunoglobulins is formed by six structurally hypervariable peptideloops, also called complementarity determining regions (CDRs). Bothvariable domains, VH and VL, contribute three CDRs to the antigenbinding site. Both of the variable domains consists each of two β-sheetstructures arranged in a layerlike fashion, which form the structurallyconserved framework bearing the hypervariable peptide loops. In this wayan inner and an outer ring of β-strands arise in the Fv fragment,wherein two CDRs are fixed between neighbouring strands of the innerring and four CDRs are fixed between strands of the inner and the outerring. In contrast, the ligand-binding sites of the lipocalins areconstructed more simply. In this case only one ring of 8 antiparallelβ-strands exists: the β-barrel. This cyclic β-pleated sheet structure isconserved in the protein fold of the lipocalins. The binding site isformed in the entry region of the β-barrel by the four peptide loops,each of which connects two neighbouring β-strands with one another.These peptide loops can vary significantly in their structure betweenthe individual members of the lipocalin family.

Despite the apparent analogy in the structure of the immunoglobulins andthe lipocalins, i.e. conserved framework regions on the one hand andhypervariable, specificity-determining segments on the other, thereexists one essential difference between these two proteins classes.Namely, while approximately 100 million different antibodies circulatein the human body and are continually generated, the same organismproduces only a few lipocalins, such as for example Rbp or ApoDmentioned above. Whereas antibodies with new antigen specificitiesconstantly arise in the immune system of a mammal through somatic generecombination and mutation, the lipocalins have in contrast remainedmostly conserved in the course of evolution in the structure andfunction of their respective ligand-binding sites. Rbp, the amino acidsequence of which is known from different organisms, serves as anexample of this. Sequence comparison of human Rbp (SWISS-PROT DatabankAccess Number P02753) with, for example, porcine Rbp (SWISS-PROTDatabank Access Number P27485) and with bovine Rbp (SWISS-PROT DatabankAccess Number P18902) reveals the presence of only 13 and 14differences, respectively. In addition, all of these amino acidsubstitutions are located in the spatial structure distant from thebinding site for retinol (see FIG. 13 in the publication of Crown etal., supra).

In the method according to the invention this gap between the functionalcharacteristics of antibodies and lipocalins is closed in that one ormore of the four peptide loops forming the ligand-binding site of alipocalin is subjected to mutagenesis, followed by chosing, i.e.selecting those protein variants (muteins), which exhibit the desiredbinding activity for a given ligand. The lipocalin muteins obtained inthis way are termed anticalins.

The following explains by way of example, namely by way of Bbp, what isto be understood by the term peptide loops in this invention in view ofthe polypeptide sequences. The four peptide loops of the lipocalinswhich, according to the inventive production of the anticalins, aremodified in their sequence by mutagenesis, are characterized by thosesegments in the linear polypeptide sequence comprising amino acidpositions 28 to 45, 58 to 69, 86 to 99 and 114 to 129 of Bbp. Each ofthese sequence segments begins before the C-terminus of one of theconserved β-strands at the open side of the β-barrel, includes theactual peptide hairpin, and ends after the N-terminus of the likewiseconserved β-strand which follows in the sequence.

With the help of published sequence alignments or those which areperformable by the person skilled in the art, or of structuralsuperpositions, the definition of the sequence positions given for Bbpcan be assigned to other lipocalins. For example, one can read off thesequence alignments reproduced in FIG. 3, which correspond to thepublished alignment of Peitsch and Boguski (New Biologist 2 (1990),197-206), that the four peptide loops in the case of ApoD include theamino acid positions 28 to 44, 59 to 70, 85 to 98 and 113 to 127. It isalso possible to identify the corresponding peptide loops in newlipocalins which are suitable for an inventive mutagenesis in the sameway.

In some cases, the relatively weak sequence homology of the lipocalinsmay prove to be problematic in the determination of the conservedβ-strands. It is therefore crucial that the polypeptide sequence becapable of forming the cyclic β-pleated sheet structure made of 8antiparallel β-strands. This can be determined by employing methods ofstructural analysis such as protein crystallography or multidimensionalnuclear magnetic resonance spectroscopy.

In other lipocalins, such as for example ApoD or Rbp, the sequencesegments suitable for mutagenesis can easily be longer or shorter thanthat of Bbp (see FIG. 3) based on the individually varying structure ofthe peptide loops. It can even be advantageous to additionally modifythe length of sequence segments by deletion or insertion of one or moreamino acids. In a preferred embodiment of the invention, those aminoacid positions in these sequence segments are mutated which correspondto sequence positions 34 to 37, 58, 60, 69, 88, 90, 93, 95, 97, 114,116, 125, and 127 of Bbp. These amino acid positions are emphasized inFIGS. 1B and 3. Correspondingly, in the case of ApoD, the sequencepositions 34 to 37, 59, 61, 70, 87, 89, 92, 94, 96, 113, 115, 123 and125 are preferred for mutagenesis. However, for the production ofanticalins not all of the sequence positions given here have to besubjected to mutagenesis.

Of course, other lipocalins besides those examples cited here are alsosuitable as an underlying structure for the production of anticalins.Preferably, the lipocalins Rbp, Bbp or ApoD, which presently havealready been exhaustively studied biochemically, are used. The use oflipocalins of human origin is especially preferred for the production ofanticalins. This especially applies when an application of the resultinganticalin(s) is intended for humans since, for example in diagnostic ortherapeutic applications in vivo, a minimumal immunogenic effect is tobe expected as compared to lipocalins from other organisms. However,other lipocalins as well as lipocalins which, possibly, have yet to bediscovered can prove to be especially advantageous for the production ofanticalins. Artificial proteins with a folding element which isstructurally equivalent to the β-barrel of the lipocalins can also beused.

Preferably the anticalins according to the invention should be able tobind the desired ligand with a determinable affinity, i.e. with anaffinity constant of at least 10⁵ M⁻¹. Affinities lower than this aregenerally no longer exactly measurable with common methods and aretherefore of secondary importance for practical applications. Especiallypreferred are anticalins which bind the desired ligand with an affinityof at least 10⁶ M⁻¹, corresponding to a dissociation constant for thecomplex of 1 μM. The binding affinity of an anticalin to the desiredligand can be measured by the person skilled in the art by a multitudeof methods, for example by fluorescence titration, by competition ELISAor by the technique of surface plasmon resonance.

The lipocalin cDNA, which can be produced and cloned by the personskilled in the art by known methods, can serve as a starting point formutagenesis of the peptide loop, as it was for example described for Bbp(Schmidt and Skerra, Eur. J. Biochem. 219 (1994), 855-863).Alternatively, genomic DNA can also be employed for gene synthesis or acombination of these methods can be performed. For the mutagenesis ofthe amino acids in the four peptide loops, the person skilled in the arthas at his disposal the various known methods for site-directedmutagenesis or for mutagenesis by means of the polymerase chainreaction. The mutagenesis method can, for example, be characterized inthat mixtures of synthetic oligodeoxynucleotides, which bear adegenerate base composition at the desired positions, can be used forintroduction of the mutations. The implementation of nucleotide buildingblocks with reduced base pair specificity, as for example inosine, isalso an option for the introduction of mutations into the chosensequence segment or amino acid positions. The procedure for mutagenesisof ligand-binding sites is simplified as compared to antibodies, sincefor the lipocalins only four instead of six sequencesegments—corresponding to the four above cited peptide loops—have to bemanipulated for this purpose.

In the methods of site-directed random mutagenesis implementingsynthetic oligodeoxynucleotides, the relevant amino acid positions inthe lipocalin structure which are to be mutated can be determined inadvance. The ideal selection of the amino acid positions to be mutatedcan depend on the one hand on the lipocalin used, and on the other handon the desired ligand. Here, it can be useful to maintain the totalnumber of mutated amino acid positions within a single experiment lowenough such that the collection of variants obtained by mutagenesis,i.e. the so-called library, can in its totality or, at least in arepresentative selection therefrom, be realized as completely aspossible in its combinatorial complexity, not only at the level of thecoding nucleic acids, but also at the level of the gene products.

It should be possible to choose the amino acid positions to be mutatedin a meaningful way especially when structural information existspertaining to the lipocalin itself which is to be used, as in the casewith Rbp and Bbp or at least pertaining to a lipocalin with a similarstructure, as for example in the case of ApoD. The set of amino acidpositions chosen can further depend on the characteristics of thedesired ligand. In the case of the small hapten-like ligand, it can forexample be useful to subject especially those amino acid positions atthe center of the ligand binding pocket to mutation, in other wordsthose in or nearby the region of the β-barrel. In contrast, in the caseof a larger antigen-like ligand, mutagenesis should also effect thoseamino acid positions in the peptide loops which are arranged in anexposed manner on the protein surface, and which are located more in themiddle of the corresponding sequence segments. Apart from such afunctional view, it can also prove advantageous to exclude single aminoacid positions in the region of the ligand-binding pocket frommutagenesis if these for example prove to be essential for the foldingefficiency or the folding stability of the protein.

One of the many applicable methods for the introduction of mutations inthe region of the four peptide loops of a lipocalin is based on the useof four oligodeoxynucleotides, each of which is derived from one of thefour corresponding sequence segments to be mutated. In the production ofthese oligodeoxynucleotides, the person skilled in the art can employmixtures of nucleic acid building blocks for the synthesis of thosenucleotide triplets which correspond to the amino acid positions to bemutated, so that codons or anticodons randomly arise for all amino acidsor, according to the genetic code and to the composition of thismixture, for a selection of the desired amino acids at this position.

For example, the first oligodeoxynucleotide corresponds in itssequence—apart from the mutated positions—at least partially to thecoding strand for the peptide loop, which is located in the polypeptidesequence of the lipocalin at the most N-terminal position. Accordingly,the second oligodeoxynucleotide corresponds at least partially to thenon-coding strand for the second sequence segment following in thepolypeptide sequence. The third oligodeoxynucleotide corresponds in turnat least partially to the coding strand for the corresponding thirdsequence segment. Finally, the fourth oligodeoxynucleotide correspondsat least partially to the non-coding strand for the fourth sequencesegment. A polymerase chain reaction can be performed with therespective first and second oligodeoxynucleotide as well as with therespective third and fourth oligodeoxynucleotide by using the nucleicacid which codes for the lipocalin and/or its complementary strand as atemplate.

The amplification products of both of these reactions can be combined byvarious known methods into a nucleic acid which comprises the sequencefrom the first to the fourth sequence segment, and which bears themutation at the chosen amino acid position. To this end, both of theproducts can for example be subjected to a new polymerase chain reactionusing flanking oligodeoxynucleotides as primers as well as one or moremediator nucleic acid molecules which contribute the sequence betweenthe second and the third sequence segment. This procedure isschematically reproduced in FIG. 4. In the choice of the number of theoligodeoxynucleotides used for the mutagenesis and their arrangementwithin the gene sequence of the lipocalin, the person skilled in the artfurthermore has numerous alternatives at his disposal.

The nucleic acid molecules which code for the sequence region with thefour peptide loops of a lipocalin and which contain mutations at thechosen positions can be connected by ligation with the missing 5′- and3′-sequences of a nucleic acid coding for the lipocalin, and can becloned in a known host organism. A multitude of procedures are at one'sdisposal for the ligation and the cloning. For example, in the course ofan amplification, synthetic nucleic acid molecules with restrictionendonuclease recognition sequences, which are also present at thecorresponding positions in the nucleic acid sequence for the lipocalin,can be attached at both ends of the nucleic acid to be cloned so that aligation is made possible following hydrolysis with the correspondingrestriction enzyme.

The present invention relates also to the directed mutagenesis of singleamino acid positions within or without the four peptide loops, forexample in order to simplify the subcloning of the mutated lipocalingene or its parts by incorporating cleavage sites for certainrestriction enzymes. For example the mutations Asn21 to Gln and Lys135to Met can be introduced in the Bbp gene in order to simplify thecloning of the mutated gene segment via two new BstXI restriction sitesat these positions. The present invention also relates to the directedintroduction of mutations within or without the four peptide loops inorder to improve certain characteristics of the anticalin, for exampleits folding stability or folding efficiency or its resistance toproteases. For example, cleavage of Bbp into two fragments, whichotherwise arises upon its production in E. coli, is thus suppressed bythe amino acid substitution Lys87 to Ser. In addition, anoligomerization of the original Bbp can be avoided by the mutation ofAsn1 to Asp. Also, by exchange of Cys116 to Ser in ApoD, its covalentcrosslinking with other proteins can be prevented and its monomericstructure can be stabilized.

In a preferred embodiment of the invention, the Bbp variant with thesubstitution Lys87 to Ser serves correspondingly as the basic structurefor the production of anticalins. Bbp variants with the substitutionsAsn1 to Asp, Asn21 to Gln, Lys135 to Met and Lys87 to Ser are especiallypreferred for use in the production of anticalins.

Longer sequence segments within the gene coding for the lipocalin canalso be subjected to random mutagenesis via known methods, for exampleby use of the polymerase chain reaction under conditions of increasederror rate, by chemical mutagenesis or by using bacterial mutatorstrains (Low et al., J. Mol. Biol. 260 (1996), 359-368). Such methodscan also be used for the further optimization of the ligand affinity orligand specificity of an anticalin which has already been produced.Mutations which possibly occur outside of the four loop regions canoften be tolerated or can even prove advantageous if they for examplecontribute to an improved folding efficiency or folding stability of theanticalin.

After having brought the coding nucleic acid sequences subjected tomutagenesis to expression, the clones carrying the genetic informationfor anticalins which bind a given ligand can be selected from thediffering clones of the library obtained. Known expression strategiesand selection strategies can be implemented for the selection of theseclones. Methods of this sort have been described in the context of theproduction or the engineering of recombinant antibody fragments, such asthe “phage display” technique or “colony screening” methods (Skerra etal., Anal. Biochem. 196 (1991), 151-155).

An embodiment of the “phage display” technique (Hoess, supra; Wells andLowman, supra; Kay et al., Phage Display of Peptides and Proteins—ALaboratory Manual (1996), Academic Press) is given here as an example ofan selection method according to the invention for anticalins with thedesired binding characteristics. The various other possible embodimentsof the “phage display” technique are hereby incorporated into thedisclosure by reference. For the exemplary selection method, phasmidsare produced which effect the expression of the mutated lipocalinstructural gene as a fusion protein with a signal sequence at theN-terminus, preferably the OmpA-signal sequence, and with the coatprotein pIII of the phage M13 (Model and Russel, in “TheBacteriophages”, Vol. 2 (1988), Plenum Press, New York, 375-456) orfragments of this coat protein, which are incorporated into the phagecoat, at the C-terminus. The C-terminal fragment ApIII of the phage coatprotein, which contains only amino acids 217 to 406 of the natural coatprotein pIII, is preferably used to produce the fusion proteins.Especially preferred is a C-terminal fragment from pIII in which thecysteine residue at position 201 is missing or is replaced by anotheramino acid.

The fusion protein can contain other components such as for example anaffinity tag or an epitope sequence for an antibody which allows theimmobilization or the later purification of the fusion protein or itsparts. Furthermore, a stop codon, can be located between the regioncoding for the lipocalin or anticalin and the gene segment for the coatprotein or its fragment, which stop codon, preferably an amber stopcodon, is at least partially translated into an amino acid duringtranslation in a suitable suppressor strain.

Phasmids here denote plasmids which carry the intergenetic region of afilamentous bacterial phage, such as for example M13 or f1 (Beck andZink, Gene 16 (1981), 35-58) or a functional part thereof, so thatduring superinfection of the bacterial cells with a helper phage, forexample M13K07, VCS-M13 or R408, one strand of the circular phasmid DNAis packaged with coat proteins and is exported into the medium asso-called phagemid. On the one hand this phagemid has the lipocalinmutein encoded by the respective phasmid built into its surface as afusion with the coat protein pIII or its fragment, wherein the signalsequence of the fusion protein is normally cleaved off. On the otherhand it carries one or more copies of the native coat protein pIII fromthe helper phage and is thus capable of infecting a recipient—generallya bacterial strain carrying an F- or F′-plasmid. In this way a physicalcoupling is ensured between the packaged nucleic acid carrying thegenetic information for the respective lipocalin mutein or anticalin,and the encoded protein which is at least partially presented infunctional form on the surface of the phagemid.

The vector pBBP20 (FIG. 5) can for example be used in the constructionof the phasmisd with the sequences coding for the Bbp muteins. Ananalogeous vector is produced for the selection of anticalins startingfrom another lipocalin, in that the DNA-sequence which codes for thislipocalin or its mutein is inserted into the vector pBBP20 instead ofthe sequence coding for Bbp. In the case of Bbp or its muteins, thenucleic acid coding for the peptide loops can for example be insertedinto the vector pBBP20 via both of the BstXI-restriction sites.Recombinant phasmids are incorporated by transformation into the E. colistrain, for example XL1-blue (Bullock et al., BioTechniques 5 (1987),376-379) or TG1. In this way, clones are made which can produce manydifferent lipocalin muteins as fusion proteins.

This library, i.e. the collection of the clones obtained, issubsequently superinfected in liquid culture according to known methodswith an M13-helper phage. After this infection the incubationtemperature of the culture can be reduced for production of thephagemids. Preferred incubation temperatures are those in which theoptimal folding of the Iipocalin mutein as a component of the fusionprotein with the phage coat protein or its fragment is expected. Duringor after the infection phase the expression of the gene for the fusionprotein with the lipocalin mutein can be induced in the bacterial cells.The induction conditions are chosen such that a substantial fraction ofthe phagemids produced presents at least one lipocalin mutein. Thephagemids are isolated after a culture incubation phase of for example 6to 8 hours. Various methods are known for isolation of the phagemids,such as for example precipitation with polyethylene glycol.

The isolated phasmids can be subjected to a selection by incubation withthe desired ligand, wherein the ligand is present in a form allowing atleast a temporary immobilization of those phagemids carrying anticalinswith the desired binding activity as fusion proteins in their coat.Among the various embodiments known to the person skilled in the art,the ligand can for example be conjugated with a carrier protein such asserum albumin and be bound via this carrier protein to a protein bindingsurface, for example polystyrene. Microtiter plates suitable for ELISAtechniques or so-called “immuno-sticks” can preferably be used for thisimmobilization of the ligand. Alternatively, conjugates of the ligandcan also be implemented with other binding groups such as for examplebiotin. The ligand can then be immobilized on surfaces which selectivelybind this group, such as for example microtiter plates or paramagneticparticles coated with streptavidin or avidin.

Protein-binding sites present on the surfaces which are charged withligands can be saturated with blocking solutions known forELISA-methods. The phagemids are for example subsequently brought incontact in a physiological buffer with the ligand immobilized on thesurface. Unbound phagemids are removed by multiple washings. Thephagemid particles remaining on the surface are subsequently eluted. Forelution, the free ligand can be added as a solution. But the phagemidscan also be eluted by addition of proteases or under moderatelydenaturing conditions, for example in the presence of acids, bases,detergents or chaotropic salts. A preferred method is the elution usingbuffers of pH 2.2, wherein the eluate is subsequently neutralized.

Afterwards, E. coli cells are infected with the eluted phagemids usinggenerally known methods. The nucleic acids can also be extracted fromthe eluted phagemids and be incorporated into the cells in anothermanner. Starting from the E. coli clones obtained in this way, phagemidsare in turn generated by superinfection with M13-helper phages accordingto the method described above and the phagemids propagated in this wayare once again subjected to a selection on the surface with theimmobilized ligand. Multiple selection cycles are often necessary inorder to obtain the phagemids with the anticalins in enriched form. Thenumber of selection cycles is preferably chosen so that in thesubsequent functional analysis at least 0.1% of the clones studiedproduce lipocalin muteins with detectable or determinable affinity forthe given ligand. Depending on the size, i.e. the complexity, of thelibrary employed, 2 to 8 cycles are typically required to this end.

For the functional analysis of the selected muteins, an E. coli strainis infected with the phagemids obtained from the selection cycles andthe corresponding double stranded phasmid DNA is isolated. Starting fromthis phasmid DNA or also from the single-stranded DNA extracted from thephagemids, the nucleic acid sequences of the selected lipocalin muteinscan be determined by the methods common for this purpose and the aminoacid sequence can be derived therefrom. The mutated region or thesequence of the entire lipocalin mutein or anticalin can be subcloned inanother expression vector and expressed in a suitable host organism.pBBP21 can for example be used as the expression vector and theexpression with pBBP21 derivatives can be performed in E. coli strains,for example E. coli-TG1. The anticalins produced by genetic engineeringcan be purified by various proteinchemical methods. The anticalinsproduced for example with pBBP21 carry the affinity peptide Strep-Tag II(Schmidt et al., J. Mol. Biol. 255 (1996), 753-766) at their C-terminusand can therefore preferably be purified by streptavidin affinitychromatography.

The selection can also be carried out by means of other methods. Manycorresponding embodiments are known to the person skilled in the art orare described in the literature. A combination of methods can also beapplied. For example clones selected or at least enriched by “phagedisplay” can additionally be subjected to a “colony screening”. Thisprocedure has the advantage that individual clones can directly beisolated with respect to the production of anticalins with detectablebinding affinity for a ligand.

In addition to the use of E. coli as host organism in the “phagedisplay” technique or the “colony screening” method, other bacterialstrains, yeast or also insect cells or mammalian cells can for examplebe used for this purpose. In addition to the selection of an anticalinfrom a primary library produced starting from a coding nucleic acidsequence for a lipocalin, comparable methods can also be applied inorder to optimize an anticalin with respect to the affinity orspecificity for the desired ligand by repeated, optionally limitedmutagenesis of its coding nucleic acid sequence.

It is surprising that anticalins can be isolated with the methodaccording to the invention which show high affinity to a given ligand.Binding constants of more than 10⁶ M⁻¹ were determined for variousfluorescein derivatives with the anticalins described in the examples.These affinity values are of the same order of magnitude as theaffinities of the lipocalins to their natural ligands, for example ofRbp to vitamin A (Cogan et al., Eur J. Biochem. 65 (1976), 71-78). Incontrast to the natural lipocalin ligands, which are normallywater-insoluble and chemically unstable, fluorescein is a relativelyhydrophilic compound, which has also been used in immunological studiesas a hapten with model character (Voss, Fluorescein Hapten: AnImmunological Probe (1984), CRC Press). Moreover fluorescein bears nostructural relationship whatsoever to biliverdin IX_(γ), the originalligand of Bbp.

Such affinities attainable with the anticalins for novel ligands arecomparable with the affinities which are known for antibodies from thesecondary immune response. Furthermore, there additionally exists thepossibility to subject the anticalins produced to a further, optionallypartial random mutagenesis in order to select variants of even higheraffinity from the new library thus obtained. Corresponding procedureshave already been described for the case of recombinant antibodyfragments for the purpose of an “affinity maturation” (Low et al.,supra; Barbas and Burton, Trends Biotechnol. 14 (1996), 230-234) and canalso be applied to anticalins in a corresponding manner by the personskilled in the art.

Surprisingly it furthermore turned out that the four peptide loopsforming the ligand binding pocket of the lipocalins exhibit hightolerance for amino acid substitutions, without the folding of thepolypeptide chain in the isolated anticalins being substantiallyaffected by it. Accordingly, it is possible to generate anticalins whichhave binding pockets with diverse surface properties so that themolecular recognition of a wide variety of ligands, also of peptides orpolypeptides as well as other macromolecules, can be realized.

If the genetic information for an anticalin has first been obtained orits amino acid sequences is known, then it can be produced withgenerally known genetic engineering methods. Preferred are methods forthe production of anticalins, wherein the anticalin, a fragment of theanticalin or a fusion protein of the anticalin and another polypeptideis produced in a bacterial or eucaryotic host organism by means ofgenetic engineering methods starting from the nucleic acid coding forthe anticalin, and is isolated from this host organism or its culture.The fact that, in doing so, normally only one structural gene has to bebrought to expression represents a significant simplification incomparison to antibodies or their fragments.

A multitude of host organisms, such as E. coli and other gram negativeor also gram positive bacteria, yeast and other eucaryotic cells can beemployed for the recombinant production. The choice between variousexpression strategies is also possible. For example, secretion with asuitable signal sequence in the host organism E. coli, as described inthe examples, leads to the correctly folded, functional protein in whichthe disulfide bonds are formed. On the other hand it is also possible toproduce an anticalin in the cytosol of a bacterial cell and, in the casethat the lipocalin is not functionally folded in the cytosol, tofunctionally refold it later in vitro. Even a refolding from aggregateswhich accumulate during the secretion in the periplasm of the bacteriumis possible.

Anticalins made by genetic engineering can be purified by means of amultitude of established methods. The suitability of the method dependson the respective host organism used, the expression strategy and otherfactors which are known to the person skilled in the art experienced inthe expression and purification of recombinant proteins. Thepurification can optionally be simplified in that the anticalin is fusedwith one or more peptide sequences. For the fusion, such peptides orproteins are preferably used which confer on the resulting recombinantprotein an affinity to certain column materials. Such fusions should notnegatively influence the function of the anticalin or must for examplebe cleavable by the insertion of a suitable protease clipping site.Typical examples for fusion partners given here are oligohistidine-tags,the Strep-Tag or the Strep-Tag II, glutathione S-transferase,maltose-binding protein or the albumin-binding domain of protein G.Anticalins can just as well be purified via their respectiveligand-binding sites by means of affinity chromatography on a columnmatrix with the immobilized matching ligand, or suitable derivatives ofthis ligand. Compared to recombinant antibody fragments, the fact thatthe anticalins are composed of a single polypeptide chain isadvantageous in the purification since no precautions have to beundertaken in order to ensure the intact association of subunits.

The structure of an anticalin can be additionally modified for thepurpose of improved production, purification or applicability. Forexample, the N— or the C-terminal peptide segment not part of theβ-barrel structure can be removed. Disulfide bonds present can beeliminated by substitution of the cysteine residues or new disulfidebonds can be introduced at another site. Free cysteine residues, such asthe residue 116 in ApoD, can be removed when they interfere with, forexample, the production or the stability of the anticalin. Optionally,cysteine residues can also be newly introduced in order to prepare, forexample, corresponding protein conjugates by chemical coupling withother components. Binding sites for further ligands, such as for examplemetal ions, can also be built into the anticalin outside of the actualligand-binding pocket. Finally, fusion proteins of anticalins and otherpolypeptides, proteins or protein domains can be made for other purposesthan protein production or purification by means of methods known to theperson skilled in the art. Preferably, the fusion can take place at theN-terminus or also at the C-terminus of the anticalin.

Such fusions can be suitable to confer new characteristics on theanticalin, such as for example enzymatic activity or affinity for othermolecules such as proteins, macromolecules or low molecular weightligands. For example, fusions with enzymes are possible which catalysechromogenic or fluorogenic reactions or which can serve for theliberation of cytotoxic agents. Further examples of fusion partnerswhich can be advantageous in practice are binding domains such as thealbumin-binding domain of protein G, protein A, antibody fragments,oligomerizing domains, toxins or also anticalins with different or thesame ligand specificity. Alternatively to the production of the fusionproteins, conjugates of anticalins and proteins, nucleic acids or almostany biomolecules and chemical compounds can be made by means of methodsknown to the person skilled in the art.

Similar to antibodies or their fragments, anticalins and theirderivatives can be employed in many areas. Anticalins are preferablyused for binding to a solid phase, so that the ligand of the anticalinor a conjugate or fusion protein of this ligand can be immobilized orseparated. Further preferred is the use of anticalins for labelling withan enzyme, an antibody or a radioactive substance or another group witha biochemical activity or with defined binding characteristics, so thatthe ligand of the anticalin or a conjugate or fusion protein of thisligand can be detected or brought in contact with it. Anticalins canserve for example in the verification of chemical structures by means ofestablished bioanalytic methods such as ELISA or Western Blot, inmicroscopy or immunosensorics. Here, the detection signal can either begenerated directlyby use of a suitable anticalin conjugate or anticalinfusion protein, or indirectly with detection of the bound anticalin bymeans of an antibody directed against it or for example by using anaffinity tag.

Preferred ligands for anticalins are on the one hand chemical compoundsin free or conjugated form which exhibit the characteristics of animmunological hapten, and on the other hand peptides, polypeptides orother macromolecules as well as corresponding conjugates thereof. Aninteresting area of application is the use of anticalins in order todetect non-radioactively labelled biomolecules, especially nucleicacids. For example, chemically reactive fluorescein derivatives for thelabelling of proteins and nucleic acids are commercially available, andmethods are also known for the incorporation of fluorescein groupsduring the synthesis or replication of nucleic acids. Correspondinglymodified nucleic acids can be used as specific gene probes and can besubsequently detected with the anticalins described in the examples.

Anticalins can also exert a quenching effect on the fluorescence of theligands bound by them, such as for example fluorescein. For suchanticalins, there arise promising possible applications in biophysicalstudies. For example, the fluorescence quenching effect of an anticalincan be exploited in a similar manner as with certain antibodies againstfluorescein in order to determine the orientation of a membrane proteinlabelled with fluorescein in the membrane bi-layer. A further area ofapplication is in the studying of the dynamics of ligand/receptorinteractions on the cell surface, wherein the ligand is labelled with afluorescein group: Various other applications in which the fluorescencequenching of fluorescein or other fluorescing compounds plays a role arealso possible. For example, an anticalin can be employed in order toavoid a disturbing background intensity by excess fluorescein-labelledreagent in the fluorescence microscopy of cells.

There exist numerous possible applications for the anticalins inmedicine. In addition to their use in diagnostics, anticalins can alsobe prepared which bind for example tissue- or tumor-specific cellularsurface molecules. Corresponding anticalins can be employed inconjugated form or as fusion proteins for “tumor imaging” or directlyfor cancer therapy. In making such anticalins it can be useful to startfrom a human lipocalin, such as for example Rbp or ApoD. In doing so,the small size of the anticalins or their derivatives has novel andadvantageous characteristics as compared to antibodies.

The invention is further illustrated by the following examples and theatteched drawings in which:

FIG. 1 schematically represents the three-dimensional molecularstructure of Bbp with its ligand biliverdin IX_(γ) (A) and depicts thespatial position of those amino acids (B) which are preferably theobject of the mutagenesis for the preparation of anticalins;

FIG. 2 compares the topography of the polypeptide chain for the ligandbinding sites of antibodies (A) and of lipocalins (B) with each other;

FIG. 3 aligns the amino acid sequence of various lipocalins;

FIG. 4 schematically illustrates the production of the library oflipocalin muteins at the nucleic acid level;

FIG. 5 schematically depicts the phasmid vector pBBP20;

FIG. 6 schematically depicts the expression vectors pBBP21 (A) andpBBP22 (B);

FIG. 7 demonstrates the binding of a peptide by anticalins in an ELISA;

FIG. 8 depicts the complexation and fluorescence quenching of the ligandfluorescein by an anticalin in a fluorescence titration.

FIG. 1 shows the crystal structure of Bbp (File 1BBP from the BrookhavenProtein Databank; Molecule A), which was graphically represented withthe help of the program MOLSCRIPT (Kraulis, J. Appl. Cryst. 24 (1991),946-950). In (A) the bound ligand as well as disulfide bonds in thepolypeptide are depicted as “ball and stick” (carbon: black; nitrogen,sulfur: dark grey; oxygen: light grey). The individual β-strands aredepicted as ribbons and the α-helix is depicted as a spiral. Thecup-like shape of the ligand-binding site can be recognized at the topon the upper end of the β-barrel formed by the eight antiparallelβ-strands. In (B) the C^(α)-positions of the amino acids are depicted inconnection with one another along the polypeptide chain. The N— andC-terminus of the polypeptide is labelled. The black-coloredC^(α)-positions are designated with the sequence numbers and denote thepositions of the mutated amino acids in the structure of Bbp accordingto the examples.

FIG. 2 shows a view from above (A) onto the antigen-binding site in theFv-fragment of an immunoglobulin which is formed together by thevariable domains VL and VH, and (B) onto the ligand-binding site of alipocalin. The β-strands are each arranged approximately perpendicularto the plane of the paper and are represented as bars. The six CDRS (L1,L2, L3, H1, H2, H3) in the immunoglobulin as well as the four peptideloops in the lipocalin connect each two β-strands with one another. Theother connecting segments and structural elements are omitted.

FIG. 3 shows a sequence comparison (amino acids are given in singleletter code) between the bilin-binding protein (SWISS-PROT Data BankAccess Number P09464), human apolipoprotein D (SWISS-PROT Data BankAccess Number P05090) and the retinol-binding protein (SWISS-PROT DataBank Access Number P02753) in the form of the mature polypeptide. Theeight segments in the region of the β-barrel corresponding to theconserved β-strands, which exhibit high similarity in the crystalstructures of Bbp and Rbp, are emphasized by underlining. The loopregions in which amino acids are preferably exchanged are marked belowthe sequence of Bbp by double underlining. Those positions in Bbp whichare mutated in the examples are additionally labelled with asterisks.The alignment between the sequences of Bbp and ApoD corresponds to thatin the publication of Peitsch and Boguski (New Biologist 2 (1990),197-206).

FIG. 4 schematically shows a strategy for the concerted mutagenesis of16 selected amino acid positions in the Bbp by repeated application ofthe polymerase chain reaction (PCR). For each of the four peptide loopsof the lipocalin in which the amino acids are to be mutated, anoligodeoxynucleotide was synthesized, (SEQ ID NO:1, SEQ ID NO:2, SEQ IDNO:3 and SEQ ID NO:4), wherein the respective mixtures of the buildingblocks given in the sequence protocol were employed at the mutationsites. Due to the composition chosen, from the altogether three possiblestop codons only the amber stop codon, TAG, could possibly arise at allof the mutated codons, which is translated as glutamine in the E. colisupE-strains XL1-blue or TG1 used for gene expression. For certainapplications, for example for gene expression in other bacterial strainsor organisms, such a nonsense codon, when it arises in the structuralgene for a selected anticalin, can be substituted by aglutamine-encoding codon by the person skilled in the art, for examplevia site-directed mutagenesis. A nucleic acid fragment with 159 basepairs was amplified (Step 1, A) with the primers SEQ ID NO:1 and SEQ IDNO:2 using the pBBP20-plasmid-DNA (SEQ ID NO:10) containing theBbp-structural gene as template. Parallel to this, a nucleic acidfragment with 164 base pairs was amplified (Step 1, B) with the primersSEQ ID NO:3 and SEQ ID NO:4, also using pBBP20 as template. The mixtureof both of these fragments served as template in a second amplificationstep in the presence of an oligodeoxynucleotide SEQ ID NO:5 hybridizingwith both of these fragments as well as the two flanking PCR primers SEQID NO:6 and SEQ ID NO:7, wherein a gene fragment of 371 base pairs wasobtained. This fragment contained all 16 mutated codons and wassubsequently cloned using both of the BstXI-restriction sites in thevector pBBP20. The use of these two restriction sites, the specialarrangement of which led to two non-compatible overhanging DNA-endsduring the restriction digest, enabled a particularly efficientligation. Both of these amino acid substitutions Asn21 to Gln and Lys135to Met with respect to the original sequence had already beenaccomplished in order to introduce both of the BstXI restriction sitesinto the Bbp structural gene.

FIG. 5 shows a drawing of pBBP20. This vector codes for a fusion proteinfrom the OmpA signal sequence, a modified Bbp with the four amino acidsubstitutions Asn1 to Asp, Asn21 to Gln, Lys87 to Ser and Lys135 to Met,the Strep-Tag II affinity tag and a shortened form of the coat proteinpIII from M13, comprising the amino acids 217 to 406 (pIII). Thestructural gene is subject to the transcriptional control of thetetracycline promoter/operator (tet^(p/o)) and ends at the lipoproteintranscription terminator (t_(lpp)). Further elements of the vector arethe replication origin (ori), the intergenic region of the filamentousbacterophage f1 (f1-IG), the ampicillin resistance gene (bla) coding forβ-lactamase and the tetracycline repressor gene (tetR). An amber stopcodon, which is partially read through in an amber suppressor hoststrain, is located between the coding region for Bbp with the OmpAsignal sequence and the Strep-Tag II as well as the coding region forthe truncated phage coat protein pIII. Both the BstXI-restriction sitesused for the cloning of the mutated gene cassette and the restrictionsites flanking the structural gene are labelled. A relevant segment fromthe nucleic acid sequence of pBBP20 is reproduced together with theencoded amino acid sequence in the sequence protocol as SEQ ID NO:10.The segment begins with a hexanucleotide sequence obtained by ligationof an XbaI overhang with an SpeI overhang complementary thereto, andends with the HindIII restriction site. The vector elements outside ofthis region are identical with the vector pASK75, the completenucleotide sequence of which is given in the published German patentpublication DE 44 17 598 A1.

FIG. 6 shows a drawing of pBBP21 (A) and of pBBP22 (B). pBBP21 codes fora fusion protein made of the OmpA signal sequence, a modified Bbpaccording to FIG. 5 and the Strep-Tag II affinity tag. This structuralgene is followed by the dsbC-structural gene (including its ribosomalbinding site) from E. coli (Zapun et al., Biochemistry 34 (1995),5075-5089) as a second cistron. The artificial operon formed in this wayis subject to the common transcriptional control of thetetracycline-promoter/operator (tet^(p/o)) and ends at the lipoproteintranscription terminator (t_(lpp)). All further genetic elements areidentical with pBBP20 according to FIG. 5. The overproduction of thebacterial disulfide isomerase in connection with the cosecretion cansupport the formation of the correct disulfide bridges in the lipocalinand can thus increase the yield of correctly folded polypeptide.However, the production of the lipocalin or of the anticalins is alsopossible without this precaution. A relevant segment from the nucleicacid sequence of pBBP21 is reproduced together with the encoded aminoacid sequence in the sequence protocol as SEQ ID NO:11. The segmentbegins with the XbaI restriction site and ends with a hexanucleotidewhich was obtained by ligation of a blunt end with a filled-in HindIIIend, whereby the original HindIII cleavage site was lost. The vectorelements outside of this region are identical to the vector pASK75, thecomplete nucleotide sequence of which is given in the German patentpublication DE 44 17 598 A1. pBBP22 codes for a fusion protein made ofthe OmpA-signal sequence, a modified Bbp according to FIG. 5, theStrep-Tag II affinity tag, and an albumin-binding domain (abd) ofprotein G from Streptococcus (Kraulis et al., FEBS Lett. 378 (1996),190-194). All further genetic elements are identical with pBBP20. Arelevant segment from the nucleic acid sequence of pBBP22 is reproducedtogether with the encoded amino acid sequence in the sequence protocolas SEQ ID NO:12. The segment begins with the XbaI-restriction site andends with the HindIII restriction site. The vector elements outside ofthis region are identical with the vector pASK75, the completenucleotide sequence of which is given in the German patent publicationDE 44 17 598 A1.

FIG. 7 shows a graphical representation of the data from Example 7, inwhich a synthesized peptide epitope of the hepatitis C-virus wasdetected with the anticalins HepC1 (squares) and HepC4 (circles) in anELISA. The values obtained with Bbp (triangles) are plotted forcomparison. “C” represents the relative protein concentration withineach dilution series.

FIG. 8 shows a graphical representation of the data from Example 8, inwhich different concentrations of the anticalin FluA are added to a 1 μMsolution of fluorescein. The fluorescence intensities were measured atan excitation wavelength of 490 nm and an emission wavelength of 512 nmand were plotted against the respective total concentration of theanticalin in the mixture. The data points were finally fitted by acurve.

EXAMPLES Example 1 Production of a Library for Lipocalin Muteins

Unless otherwise indicated, genetic engineering methods known to theperson skilled in the art were used, as for example described inSambrook et al. (Molecular Cloning. A Laboratory Manual (1989), ColdSpring Harbor Press).

PCR was applied in multiple steps according to FIG. 4 for the concertedmutagenesis of in total 16 selected amino acid positions in the fourpeptide loops of Bbp. The PCR reactions were carried out in a volume of50 μl in both of the first amplification steps, wherein 10 ng pBBP20plasmid DNA were employed as template as well as 25 pmol of therespective primers, which had been synthesized according to theconventional phosphoramidite method. In addition, the reaction mixturecontained 5 μl 10× Taq buffer (100 mM Tris/HCl pH 9, 0, 500 mM KCl, 1%v/v Triton X-100), 3 μl 25 mM MgCl₂, 4 μl dNTP-Mix (2.5 mM DATP, dCTP,dGTP, dTTP). After bringing to volume with water, the mixture wasoverlayed with mineral oil and was heated to 94° C. for 2 minutes in anautomated thermocycler. Subsequently 2.5 u Taq DNA-polymerase (5 u/μl,Promega) were added and 20 temperature cycles of 1 minute at 94° C., 1minute at 60° C. and 1.5 minute at 72° C. were carried out, followed byan incubation for 5 minutes at 60° C. The desired amplification productswere isolated from Low Melting Point Agarose (Gibco BRL) by preparativeagarose gel electrophoresis using the Jetsorb DNA extraction kit(Genomed) according to the instructions of the manufacturer.

The subsequent amplification step was carried out in a 100 μl mixture,wherein approximately 6 ng of both of these respective fragments wereused as templates, and 50 pmol of each of the primers SEQ ID NO:6 andSEQ ID NO:7 as well as 1 pmol of the oligodeoxynucleotide SEQ ID NO:5.The remaining components of the PCR mixture were added in the doubleamounts as in the previous amplification steps. PCR took place with 20temperature cycles of 1 minute at 94° C., 1 minute at 55° C., 1.5minutes at 72° C., followed by a subsequent incubation for 5 minutes at60° C. The expected fragment was again isolated by preparative agarosegel electrophoresis.

For the cloning of this fragment representing the library of thelipocalin muteins in nucleic acid form, it was first cut with therestriction enzyme BstXI (New England Biolabs) according to theinstructions of the manufacturer. The purification of the nucleic acidfragment obtained (335 base pairs, bp) again took place by means ofpreparative agarose gel electrophoresis. The DNA of the vector pBBP20was analogously cut with BstXI and the larger of the two fragments (3971bp) was isolated.

For the ligation, 0.93 μg (4.2 pmol) of the PCR fragment and 11 μg (4.2pmol) of the vector fragment was incubated in the presence of 102 WeissUnits T4 DNA ligase (New England Biolabs) in a total volume of 500 μl(50 mM Tris/HCl pH 7.8, 10 mM MgCl₂, 10 mM DTT, 1 mM ATP, 50 μg/ml BSA)for two days at 16° C. The DNA was subsequently precipitated by adding10 μg tRNA from yeast (Boehringer Mannheim), 25 μl 5 M ammonium acetateand 100 μl ethanol per 24 μl of ligation mixture. Incubation at −20° C.for three days was followed by centrifugation (25 minutes, 16000 g, 4°C.). Each precipitate was washed with 200 μl ethanol (70% v/v, −20° C.)and dried under vacuum. The DNA was finally dissolved in 43.6 μl TE/10(1 mM Tris/HCl pH 8.0, 0.1 mM EDTA pH 8.0). The DNA concentration of thesolution obtained was estimated by analytical agarose gelelectrophoresis via the fluorescence intensity of the bands stained withethidium bromide in comparison to a sample of known concentration.

The preparation of electrocompetent cells of the E. coli K12 strainXL1-blue (Bullock et al., supra) took place according to the methodsdescribed by Tung and Chow (Trends Genet. 11 (1995), 128-129) and byHengen (Trends Biochem. Sci. 21 (1996), 75-76). 1 l LB-medium wasadjusted by addition of a stationary XL1-blue overnight culture to anoptical density at 600 nm of OD₆₀₀=0.08 and was incubated at 200 rpm and26° C. in a 2 l Erlenmeyer flask. After reaching an OD₆₀₀=0.6, theculture was cooled for 30 minutes on ice and subsequently centrifugedfor 15 minutes at 4000 g and 4° C. The cell sediment was washed twiceeach with 500 ml ice-cold 10% w/v glycerol and was finally resuspendedin 2 ml of ice-cold GYT-medium (10% w/v glycerol, 0.125% w/v yeastextract, 0.25% w/v tryptone).

The Easyjec T basic system (EquiBio) was used with the accompanyingcuvettes (electrode separation 2 mm) for the electroporation. All stepswere carried out in the cold room at 4° C. Each 5 to 6 μl of the DNAsolution mentioned above (245 ng μl) was mixed with 40 μl of the cellsuspension, was incubated 1 minute on ice and was finally transferred tothe cuvette. After the electroporation the suspension was immediatelydiluted in 2 ml of fresh, ice-cold SOC-medium (2% w/v tryptone, 0.5% w/vyeast extract, 10 mM NaCl, 10 mM MgSO₄ ⁻, 10 mM MgCl₂) and was shakenfor 60 minutes at 37° C. and 200 rpm. The cells were subsequently eachsedimented for 2 minutes at 3600 g, were resuspended in 1 ml LB-mediumwith 100 μg/ml ampicillin (LB/Amp) and were plated at portions of 200 μlon agar plates (140 mm diameter) with LB/Amp-medium. Bei employing intotal 10.7 μg of the ligated DNA, 3.73×10⁸ transformants were obtainedin this way with eight electroporation runs, and the transformants weredistributed onto 40 agar plates and were further used according toExample 2.

Example 2 Phagemid Presentation and Selection of Anticalins againstFluorescein

The cells plated onto LB/Amp-agar, which were transformed with thephasmid vectors coding for the library of the lipocalin muteins asfusion proteins, were incubated for 14 hours at 32° C. The colonies werethen scraped from the agar plates with respective addition of 10 ml 2×YT/Amp-medium, were transferred to a sterile Erlenmeyer flask and wereshaken for 20 minutes at 37° C., 200 rpm for complete suspension. 500 mlof 2× YT/Amp-medium prewarmed to 37° C. were inoculated with 2.3 ml ofthis suspension so that the cell density OD₅₅₀ was 0.08.

This culture was incubated at 37° C., 160 rpm to a cell density ofOD₅₅₀=0.5, was infected (multiplicity of infection approximately 10)with VCS-M13 helper phage (Strategene) and was shaken for additional 30minutes at 37° C., 160 rpm. Kanamycin (70 μg/ml) was subsequently added,the incubator temperature was lowered to 26° C. and, after 10 minutes,anhydrotetracycline was added to 25 μg/l (250 μl of a 50 μg/ml stocksolution in dimethylformamide, DMF) to induce gene expression.Incubation continued for another 7 hours at 26° C., 160 rpm.

50 ml were removed from this culture and the cells were sedimented bycentrifugation (15 minutes, 12000 g, 4° C.). The supernatant containingthe phagemid particles was sterile-filtered (0.45 μm), was mixed with ¼volumes (12.5 ml) 20% w/v PEG 8000, 15% w/v NaCl, and was incubatedovernight at 4° C. After centrifugation (20 minutes, 18000 g, 4° C.) theprecipitated phagemid particles were dissolved in 2 ml of cold PBS (4 mMKH₂PO₄, 16 mM Na₂HPO₄, 115 mM NaCl, pH 7.4). The solution was incubatedon ice for 30 minutes and was distributed into two 1.5 ml reactionvessels. After centrifugation of undissolved components (5 minutes,18500 g, 4° C.) each supernatant was transferred to a new reactionvessel.

Mixture with ¼ volumes 20% w/v PEG 8000, 15% w/v NaCl and incubation for30 to 60 minutes on ice followed in order to reprecipitate the phagemidparticles. After centrifugation (20 minutes, 18500 g, 4° C.) thesupernatant was removed and the precipitated phagemid particles weredisolved in a total of 1 ml PBS. After incubation for 30 minutes on icethe solution was centrifuged (5 minutes, 18500 g, 4° C.) and thesupernatant was used directly for the affinity enrichment.

Immuno-sticks (NUNC) were used for the affinity enrichment of therecombinant phagemids carrying the anticalin fusion proteins. These werecoated overnight with 800 μl of a conjugate from bovine serum albumin(BSA) and 4-glutarylamido-fluorescein (100 μg/ml) in PBS.

For the production of the conjugate, 4-amino-fluorescein(fluoresceinamine isomer I, Fluka) was first reacted with a 15-foldmolar excess of glutaric anhydride at pH 7.0 according to the procedureof Ogamo et al. (Carbohydrate Res. 105 (1982), 69-85), in order to laterensure the steric accessibility of the fluorescein group. The reactionproduct 4-glutarylamido-fluorescein, which carried a carboxylic acidgroup on an aliphatic side chain suitable for coupling with BSA, wassubsequently purified by recrystallization from acetone/water. Asolution of 17.3 mg (37.5 μmol) of this substance in 25 μl DMF was thenmixed with 4.31 mg (37.5 μmol) N-hydroxysuccinimide in 25 μl DMF as wellas with 7.2 mg (37.5 μmol) 1-ethyl-3-(3-dimethylaminopropyl)carbodiimidefor activation. The mixture was incubated for 1 hour at room temperature(RT). 20 μl of this solution was mixed with a solution of 10 mg BSA in980 μl 5% w/v NaHCO₃ pH 8.1 and incubated for 3 hours at RT. Afterremoval of excess reactants from the BSA-conjugate by means of a PD-10gel filtration column (Pharmacia), a coupling with 8 molecules of4-glutarylamido-fluorescein per BSA-molecule was determined viaabsorption of the fluorescein group at 495 nm (ε=72000 M⁻¹ cm⁻¹).

Unoccupied binding sites on the surface of the Immuno-Stick weresaturated by incubation with 1.2 ml 2% w/v BSA in PBST (PBS with 0.1%v/v Tween 20) for 2 hours at RT. After washing quickly three times with1.2 ml PBST each, the Immuno-Stick was incubated in a mixture of 250 μlof the phagemid solution and 500 μl of blocking buffer (2% w/v BSA inPBST) for 1 hour at RT.

For the removal of non-bound phagemids, washing followed eight times (inthe first selection) or ten times (in selection cycles 2 to 6), eachtime with 950 μl PBST for 2 minutes. Adsorbed phagemids were finallyeluted by 10 minute treatment of the Immuno-Stick with 950 μl 0.1 Mglycine/HCl pH 2.2, wherein the pH of the elution fraction wassubsequently immediately neutralized by mixing with 160 μl 0.5 M Tris.

For the amplification, this phagemid solution (1.1 ml, depending on theselection cycle between 10⁶ and 10⁸ Colony-forming Units) was shortlywarmed to 37° C., was mixed with 4 ml of an exponentially growingculture of E. coli XL1-blue (OD₅₀₀=0.5), and was incubated for 30minutes at 37° C., 200 rpm. The cells infected with the phagemids weresubsequently sedimented (2 minutes, 4420 g, 4° C.), were resuspended in800 μl of the culture medium, and were plated out onto four agar plateswith LB/Amp-medium (140 mm diameter).

For the repeated production and affinity enrichment of phagemidparticles the same prodecure as described at the beginning of thisexample was used. In these cases 50 ml 2× YT/Amp-medium was inoculatedwith 0.2 to 1 ml of the suspension of the cells grown on the agarplates. Five further selection cycles with the BSA-fluorescein conjugatewere carried out in this way.

Example 3 Production of the Anticalins

For the preparative production of the anticalins the gene cassettebetween both BstXI-cleavage sites from the pBBP20 vector was subclonedin the expression plasmid pBBP21. The Bbp originally encoded on pBBP21was also produced as a control.

For the subcloning the phasmid DNA was isolated using the QIAprep SpinMiniprep Kit (QIAGEN) from the mixture of the E. coli-cells from Example2, which were infected with the phagemids of the last selection cycle.This was cut with the restriction enzyme BstXI and the smaller of thetwo fragments (335 bp) was purified by preparative agarose gelelectrophoresis as described in Example 1. The DNA of the vector pBBP21was cut with BstXI in the same way and the larger of the two fragments(4132 bp) was isolated.

For the ligation 1.5 Weiss Units T4 DNA Ligase (Promega) were added toeach 50 fmol of both DNA-fragments in a total volume of 20 μl (30 mMTris/HCl pH 7.8, 10 mM MgCl₂, 10 mM DTT, 1 mM ATP) and this wasincubated for 5 hours at 16° C. E. coli TG1-F⁻ (E. coli K12 TG1, whichhad lost its episome through repeated culturing under non-selectiveconditions) was then transformed with 5 μl of this ligation mixtureaccording to the CaCl₂-method (Sambrook et al., supra).

The plasmid-DNA was isolated from ten of the colonies obtained and theligation was checked by restriction analysis with the enzymes HindIIIand KpnI. All ten plasmids showed the expected fragment sizes of 342 and4125 bp.

Sequence analysis of the Bbp-gene cassettes was performed by means ofthe T7 sequencing kit (Pharmacia) according to the instructions of themanufacturer using the oligodeoxynucleotides SEQ ID NO:8 and SEQ IDNO:9. In doing so, among the ten isolated plasmids, only four differentsequences were found, the gene products of which were designated FluA,FluB, FluC and FluD. The DNA sequence of FluA was present twice, that ofFluB four times, that of FluC three times, and that of FluD once. Thenucleotide sequences of FluA, FluB, and FluC were translated into aminoacid sequences and those amino acids deviating from Bbp are given intable 1.

The protein production of the corresponding clones was carried out on a50 ml scale in order to study the binding activity of the anticalins inan ELISA (Example 4). To this end, 4 ml of LB/Amp-medium were inoculatedwith a single colony of the TG1-F⁻ transformant carrying the respectiveplasmid, and was incubated overnight at 30° C., 200 rpm. 50 ml ofLB/Amp-medium were then inoculated with 500 μl of this preculture andwere shaken at 22° C., 200 rpm to an OD₅₅₀=0.5. Induction followed with200 μg/l anhydrotetracycline (50 μl of a 200 μg/ml-stock solution inDMF) followed by shaking for 3 further hours at 22° C., 200 rpm. Thecells were sedimented by centrifugation (15 minutes, 4420 g, 4° C.) andwere resuspended in 1 ml cold periplasmic release buffer (100 mMTris/HCl pH 8.0, 500 mM Saccharose, 1 mM EDTA). Incubation for 30minutes on ice followed after addition of 25 μl of a solution of 1 mg/mllysozyme to the periplasmic release buffer. The spheroplasts weresedimented by centrifugation (15 minutes, 18500 g, 4° C.) and thesupernatant was transferred into a new reaction vessel as theperiplasmatic protein extract.

For large scale protein production a 50 ml-preculture (LB/Amp-medium)was inoculated directly with a single colony of the TGl-F⁻-straintransformed with the corresponding plasmid and was shaken at 30° C., 200rpm overnight. In the case of the anticalins FluA and FluB, the E. colistrain JM83 (Yanisch-Perron et al., Gene 33 (1985), 103-119), which doesnot carry a supE gene, was used. The total preculture was used forinoculation of 2 l LB/Amp-medium in a 51-Erlenmeyer flask, after whichthe culture was incubated at 22° C., 200 rpm. Induction with 200 μg/lanhydrotetracycline (200 μl of a 2 mg/ml stock solution in DMF) wasperformed at a cell density of OD₅₅₀=0.5, followed by shaking forfurther 3 hours at 22° C., 200 rpm.

The cells were centrifuged (15 minutes, 4420 g, 4° C.) and, afterdecanting the supernatant, were resuspended in 20 ml of the periplasmicrelease buffer with cooling on ice. After addition of 50 μg/ml lysozyme(100 μl of a solution of 10 μg/ml lysozyme in the periplasmic releasebuffer) incubation followed for 30 minutes on ice. Subsequently thespheroplasts were separated in two subsequent centrifugation steps (15minutes, 4420 g, 4° C. and 15 minutes, 30000 g, 4° C.). Theperiplasmatic protein extract isolated in this way was dialyzed againstCP-buffer (100 mM Tris/HCl pH 8.0, 150 mM NaCl, 1 mM EDTA), wassterile-filtered, and was used for the chromatographic purification.

The purification took place by means of the Strep-Tag II-affinity tag(Schmidt et al., supra) fused to the C-terminus of the lipocalin mutein.In the present case the streptavidin mutein “1” was employed (GermanPatent Application 196 41 876.3; Voss and Skerra, Protein Eng. 10(1997), 975-982), which was coupled to an activated sepharose (5 mg/mlimmobilized streptavidin, relative to the bed volume of the matrix).

A 2 ml bed volume chromatography column filled with this material wasequilibrated with 10 ml CP-buffer at 4° C. and a flow rate of 20 ml/h.Chromatography was monitored by measuring the absorption at 280 nm ofthe eluate in a flow-through photometer. After the application of theperiplasmatic protein extract, the column was washed with CP-bufferuntil the base line was reached and the bound anticalin was subsequentlyeluted with 10 ml of the solution of 2.5 mM D-desthiobiotin (Sigma). Thefractions containing the purified anticalin were checked viaSDS-polyacrylamide gel electrophoresis (Fling und Gregerson, Anal.Biochem. 155 (1986), 83-88) and were pooled. The protein yields werebetween 200 μg and 3 mg per 2 l culture. TABLE 1 Sequencecharacteristics of selected Anticalins Amino Acid- Position Bbp FluAFluB FluC HepC1 HepC4 34 Asn Ser Gln Ser Lys Gln^(a) 35 Ser Pro His LysThr Ala 36 Val Asn Trp Asn Lys Pro 37 Glu Gly Asp Gly Gln^(a) Gly 58 AsnArg Arg Arg Leu Pro 60 His Asp Arg Thr His Asn 69 Ile Met His Gln^(a)Phe Ala 88 Leu Arg Val Arg Val Trp 90 Tyr Val Arg Val Ala Gly 93 Val TyrArg Lys Phe Leu 95 Lys Arg Arg Arg Ser Ala 97 Asn Thr Gly Gly Gln Trp114  Tyr Ser Arg Arg Ala Pro 116  Lys Arg Arg Arg Tyr Arg 125  Gln TrpTrp Leu Val Leu 127  Phe His His His Phe Pro  40^(b) Gly Arg Glu  68^(b)Phe Val  70^(b) Glu Lys  96^(b) Glu Lys 100^(b)   Asn Ser^(a)These glutamic acid residues were encoded by amber stop codons.^(b)These amino acid substitutions arose due to random mutations.

Example 4 Measurement of the Affinity of the Anticalins for Fluoresceinand its Derivatives

For the detection of binding in an ELISA (Enzyme-linked ImmunosorbentAssay) the wells of a microtiter plate (Micro Test III Flexible AssayPlate; Falcon) were first each filled with 100 μl of a 100 μg/mlsolution of the BSA-fluorescein conjugate from Example 2 in PBS and wereincubated overnight at RT. Unconjugated BSA served as a control. Thesolution was removed and unoccupied binding sites were saturated with200 μl 2% w/v BSA in PBST for 2 hours. After washing three times withPBST, 100 μl of the periplasmatic protein extract from the production onthe 50 ml scale (Example 3) were filled into the wells. Dilution seriesin PBST were prepared starting from this protein solution. 1 hourincubation at RT was followed again by rewashing three times with PBST,and a streptavidin-alkaline phosphate conjugate (Amersham), diluted1:1000 with PBST, was filled into the wells. This enzyme conjugateserved for the recognition of the Strep-Tag II-appendix at theC-terminus of the anticalins. Incubation was performed for 1 hour at RTand followed by washing two times with PBST and two times with PBS.Detection of the anticalins bound to the fluorescein groups finally tookplace via hydrolysis of p-nitrophenyl phosphate, catalyzed by thealkaline phosphatase. For this purpose, 100 μl of a solution of 0.5mg/ml p-nitrophenyl phosphate (Amresco) in AP-buffer (100 mM NaCl, 5 mMMgCl₂, 100 mM Tris/HCl pH 8.8) were filled into the wells and theproduct formation was monitored by measuring the absorption at 405 nm ina SpectraMax 250 photometer (Molecular Devices).

In doing so, practically no binding was detected for FluD and Bbp, whileFluA, FluB, and FluC showed intense binding signals. In comparison, thesignal was strongest for FluC, followed by FluA and FluB.

The ligand-binding characteristics of the anticalins, were thendetermined by fluorescence titration. Here, the decrease in theintrinsic tyrosine and tryptophan fluorescence of the protein uponcomplex formation with the ligand was measured. The measurements weremade with a fluorescence photometer (MS III, Photon TechnologyInternational Inc.) at an excitation wavelength of 280 nm (slit width 5nm) and emission wavelength of 340 nm (slit width 10 nm). Fluorescein,4-amino-fluorescein as well as its conjugate with glutaric acid fromExample 2 were used as ligands. These three ligands showed nosignificant inherent fluorescence at the given wavelengths.

PBS with addition of 1 mM EDTA, pH 7.4 (adjusted with NaOH) served asbuffer system. All solutions used were sterile-filtered (0.45 μm). Thesolution of the respective purified anticalin from Example 3 wasdialyzed three times against this buffer and was adjusted to aconcentration of 1 μM by dilution. The concentration was determined byabsorption at 280 nm using calculated extinction coefficients of 63680M⁻¹ cm⁻¹ for FluB as well as 52300 M⁻¹ cm⁻¹ for FluC. For FluA and Bbp,the corrected calculated extinction coefficients in the presence ofguanidinium chloride according to Gill and von Hippel (Anal. Biochem.182 (1989), 319-326) of 59755 M⁻¹ cm⁻¹ (FluA) as well as 54150 M⁻¹ cm⁻¹(Bbp) were used.

For the measurement, 2 ml of the anticalin solution was applied in aquartz cuvette equipped with a stirring bar and thermostatted at 25° C.in the sample holder of the photometer. Subsequently altogether 40 μl ofa 250 μM to 1 mM solution of the ligand in the same buffer were added bypipetting in steps of 1 μl to 4 μl. The accompanying dilution of theapplied protein solution of maximally 2% was not taken into account inthe subsequent analysis of the data. After every titration step thesample was incubated for 1 minute with stirring for equilibration, andthe fluorescence signal was measured as an average value over 10 s.After subtraction of the fluorescence value for the buffer, the signalswere normalized to a starting value of 100% and were corrected for theinner filter effect of the ligand. For this purpose, fluorescencetitrations were carried out with the respective ligand, in which theanticalin solution was replaced by N-Acetyl-L-tryptophanamide (Sigma).

The measured values of a titration series obtained in this way werefitted to the following equation by non-linear regression with the helpof the computer program Kaleidagraph (Abelbeck Software):$F = {{\left( {\lbrack P\rbrack_{t} - \lbrack L\rbrack_{t} - K_{d}} \right)\frac{f_{P}}{2}} + {\left( {\lbrack P\rbrack_{t} + \lbrack L\rbrack_{t} + K_{d}} \right)\frac{f_{PL}}{2}} + {\left( {f_{P} - f_{PL}} \right)\sqrt{\frac{\left( {\lbrack P\rbrack_{t} + \lbrack L\rbrack_{t} + K_{d}} \right)^{2}}{4} - {\lbrack P\rbrack_{t}\lbrack L\rbrack}_{t}}}}$

Here, F is the normalized fluorescence intensity and [P]_(t) theconcentration of the anticalin. [L]_(t) is the total concentration ofthe ligand in each titration step. f_(PL) and K_(d) were fitted as freeparameters to the data measured and represent the fluorescencecoefficients of the anticalin-ligand complex as well as thethermodynamic dissociation constants of this complex, respectively. Inthe case of FluC, [P]_(t) was additionally fitted as a free parameter.The dissociation constants determined for the anticalins FluA, FluB, andFluC are given in table 2. The binding effect in the measurement forcomparison with Bbp was so weak that a dissociation constant could notbe determined in this case. TABLE 2 Dissociation constants for thecomplexes of anticalins and fluorescein derivatives 4-Aminofluo-4-Glutarylami- Fluorescein rescein dofluorescein FluA 118 ± 14 nM 224 ±6 nM 601 ± 16 nM FluB  5.73 ± 0.86 μM  2.84 ± 0.3 μM  4.70 ± 0.51 μMFluC 411 ± 20 nM  299 ± 41 nM 78 ± 3 nM

Example 5 Selection of Anticalins Against a Hepatitis C-Peptide Epitope

For the selection of the anticalins, the library made in Example 1 wasused. The amplification and isolation of the phagemids took placeexactly as described in Example 2. A biotinylated synthetic hepatitisC-peptide epitope corresponding to the peptide fragment no. 59 from thesurface protein NS4 of HCV (Khudyakow et al., Virology 206 (1995),666-672) was used as peptide ligand. The peptide, SEQ ID NO:13, wassynthesized according to the common Fmoc-method using a PS3 automat(RAININ Instrument Co.), wherein Rink amide MBHA-resin (Novabiochem) wasused. Subsequent to the coupling of the amino acid building blocks fromthe C— to the N-terminus, amino caproic acid was coupled as aboc-protected derivative and, in the last step, D-biotin (Sigma) wascoupled. The peptide, cleaved from the resin and deprotected, waspurified by HPLC and its composition was checked by ESI-massspectrometry.

For the affinity enrichment of the recombinant phagemids carrying theanticalin fusion proteins, superparamagnetic particles coated withstreptavidin (Dynabeads M-280 streptavidin, Dynal) were used. The amountof the peptide ligand was adjusted such that it was on the one handpresent in molar excess relative to the phagemids employed, and that onthe other hand the binding capacity of the streptavidin for the biotingroups was not exceeded.

For this purpose, 20 μl of the peptide solution (20 μg/ml in PBS) wasmixed with 280 μl of a solution of freshly prepared phagemid (3.0×10¹²cfu/ml) and was incubated for 1 hour at RT, after which 100 μl of asolution of 8% w/v BSA, 0.4% v/v Tween 20 in PBS was added. Parallel tothis 100 μl of the commercially available suspension of the magneticparticles were washed three times each with 100 μl PBS and wereincubated with 100 μl 2% w/v BSA in PBST for 1 hour at RT in order tosaturate unspecific binding sites. After removal of the supernatant, thepeptide/phagemid mixture was added to the magnetic particles, they wereresuspended and incubated for 10 minutes at RT. To saturate free biotinbinding sites of the streptavidin, 10 μl of a solution of 4 μMdesthiobiotin in PBS were finally added to the mixture and it wasincubated for 5 minutes at RT.

For the removal of unbound phagemids, the magnetic particles were washed8 times with 1 ml each of PBST, 0.1 μM desthiobiotin. To this end themagnetic particles were collected at the wall of 1.5 ml Eppendorf tubewith the help of a magnet and the supernatant was decanted. Followingthis, the magnetic particles were resuspended with fresh buffer and wereheld in suspension for 1 minute by rotation of the vessel. The elutionof the bound phagemids took place by a 10 minute incubation of theresuspended particles in 950 μl 0.1 M glycin/HCl pH 2.2. The pH value ofthe solution was subsequently immediately neutralized by addition of 160μl 0.5 M Tris.

Subsequently the eluted phagemids were propagated as described inExample 2 and were implemented for a new affinity selection under theconditions given above. In total, 6 selection cycles were carried out.

Example 6 Identification of Deptide-Binding Anticalins by use of the“Colony Screening”-Method

For the analytical production of the anticalins as fusion proteins withthe Strep-Tag II as well as with the albumin-binding domain and theircharacterization by “colony screening”, the gene cassette between bothBstXI cleavage sites from the vector pBBP20 was subcloned into pBBP22.

For this purpose the phasmid DNA was isolated from the mixture of the E.coli clones obtained by infection with the phagemids from Example 5eluted during the last selection cycle, using the QIAprep Spin MiniprepKits (QIAGEN). The DNA was cut with the restriction enzyme BstXI and thesmaller of the two fragments (335 bp) was purified by preparativeagarose-gel electrophoresis as described in Example 1. The DNA of thevector pBBP22 was cut with BstXI and the larger of the two fragments(3545 bp) was isolated in the same way.

For the ligation, each 50 fmol of the two DNA-fragments were mixed with1.5 Weiss Units T4 DNA ligase (Promega) in a total volume of 20 μl (30mM tris/HCl pH 7.8, 10 mM MgCl₂, 10 mM DTT, 1 mM ATP) and this wasincubated overnight at 16° C. E.coli TG1-F⁻ was transformed with 5 μl ofthis ligation mixture according to the CaCl₂-method.

A hydrophilic PVDF membrane (Millipore, type GVWP, pore size 0.22 μm),labelled at one position and cut to size, was laid onto an LB/Amp agarplate and 150 μl of the cell suspension from the transformation batchwere uniformly plated out onto this membrane. The amount oftransformation batch plated out was measured such that approximately 500colonies were obtained. The plate was incubated for 6.5 hours at 37° C.in the incubation cabinet until the colonies had reached a size easilyrecognizable by the naked eye.

In the meantime a hydrophobic membrane (Millipore, Immobilon P, poresize 0.45 μm), also cut to size, was moistened with PBS according to theinstructions of the manufacturer. It was subsequently agitated for 4hours at RT in a solution of 10 mg/ml human serum albumin (HSA, Sigma)in PBS. Remaining binding sites on the membrane were saturated byincubation with 3% w/v BSA, 0.5% v/v Tween 20 in PBS for 2 hours at RT.The membrane was washed twice for 10 minutes each with 20 ml PBS andagitated afterwards for 10 minutes in 10 ml LB/Amp medium, to which 200μg/l anhydrotetracycline were added. It was subsequently marked at oneposition and was laid onto a culture plate with LB/Amp agar, whichadditionally contained 200 μg/l anhydrotetracycline. The hydrophilicmembrane on which the colonies were grown was laid onto the hydrophobicmembrane in such a way that both of the marks superimposed. The cultureplate was incubated with both membranes at 22° C. for 15 hours. Duringthis phase the respective lipocalin muteins from the colonies weresecreted and were immobilized via the albumin-binding domain on the HSAon the lower membrane.

After this, the upper membrane with the colonies was transferred to afresh LB/Amp agar plate and stored at 4° C. The hydrophilic membrane wasremoved, was washed three times for 10 minutes each with 20 ml PBST, andwas subsequently incubated 1 hour in 10 ml of a solution of 1 μM SEQ IDNO:13 in PBST. After washing twice in PBST, incubation followed for 1hour with 10 ml avidin-alkaline phosphatase conjugate(ExtrAvidin-AP-Conjugate, Sigma, dilution 1:1000 in PBST). The membranewas subsequently washed for 5 minutes each twice with PBST and twicewith PBS and agitated for 10 minutes in AP-buffer (0.1 M Tris/HCl pH8.8, 0.1 M NaCl, 5 mM MgCl₂). For the chromogenic detection reaction,the membrane was incubated in 10 ml AP-buffer, to which 30 μl BCIP (50μg/ml in dimethylformamide) and 5 μl NBT (75 μg/ml in 70% v/vdimethylformamide) were added, until distinct color signals could berecognized at the positions of some of the colonies. In this way thebinding activity for the peptide ligand of the anticalins produced bythese colonies was detected.

Eight of these colonies were cultured. The plasmid DNA was isolated andthe Bbp gene cassette was subjected to sequence analysis as in Example3. All clones exhibited different sequences. The characteristic aminoacids of the anticalins HepCl and HepC4 are given in table 1.

Example 7 Use of the Anticalins for the Detection of Hepatitis C PeptideEpitopes in a Sandwich-ELISA

Starting from the clones found in Example 6, the correspondinganticalins were produced as fusion proteins with the Strep-Tag II andthe albumin-binding domain. Gene expression took place on a 50 ml scale.To this end 4 ml each of LB/Amp medium were inoculated with a singlecolony of TG1-F⁻ carrying the respective plasmid and were incubatedovernight at 30° C., 200 rpm. 500 μl of each pre-culture were then usedfor inoculation of 50 ml LB/Amp medium and were shaken at 22° C., 200rpm to an OD₅₅₀=0.5. After that induction was performed with 200 μg/lanhydrotetracycline (50 μl of a 200 μg/ml stock solution in DMF),followed by shaking for 3 further hours at 22° C., 200 rpm. The cellswere sedimented by centrifugation (15 minutes, 4420 g, 4° C.) and werere-suspended in 1 ml each of cold periplasmic release buffer (100 mMTris/HCl pH 8.0, 500 mM saccharose, 1 mM EDTA). After addition of 25 μlof a solution of 1 mg/ml lysozyme to the periplasmic release buffer,incubation followed for 30 minutes on ice. The spheroplasts weresedimented by centrifugation (15 minutes, 18500 g, 4° C.) and thesupernatant was transferred to a new reaction vessel as theperiplasmatic protein extract.

For the ELISA, the wells in a microtiter plate (ELISA-STRIP, 2×8 wells,KO, F-form, high binding capacity, Greiner) were each filled with 200 μlof a solution of 20 mg/ml HSA in 50 mM NaHCO₃ pH 9.6 and were incubatedfor 1 hour at RT. After removal of the solution, the unoccupied bindingsites were saturated with 200 μl 3% w/v BSA in PBS with 0.5% v/v Tween20 for 1 hour. After washing three times with PBST, 50 μl of theundiluted periplasmic protein extract were filled into the respectivefirst well of a row. 50 μl PBS were first placed into each of thesubsequent wells of each row. 50 μl of the periplasmic protein extractwere then pipetted into the respective second well, were mixed, and,starting from this, 1:2 dilutions in the subsequent wells were preparedin a stepwise manner. The periplasmic protein extract with the Bbp,which was made using pBBP22 as expression plasmid, served as a control.

After 1 hour incubation at RT washing followed three times with PBST,followed by pipetting of 200 μl of the ligand solution (SEQ ID NO:13, 1μM in PBST) into each well. After 1 hour incubation at RT, washingfollowed with PBST and, after that, 50 ml avidin-alkaline phosphataseconjugate (ExtrAvidin-AP-Conjugate, Sigma), diluted 1:1000 in PBST, wereadded to each well. Incubation followed for 1 hour at RT, withsubsequent washing twice with PBST and twice with PBS. The detection ofthe bound anticalin took place by means of a chromogenic reaction in thepresence of p-nitrophenylphosphate. To this end, 100 μl of a solution of0.5 mg/ml p-nitrophenylphosphate (Amresco) in AP-buffer were placed ineach well and product formation was measured by absorption at 405 nm ina SpectraMax 250 photometer (Molecular Devices) as dA/dt number.

In the case of Bbp, only low signals were detectable, whereas allanticalins analyzed showed clear binding. The signal for HepCl wasstrongest, followed by HepC4. The binding curves for HepCl, HepC4 andBbp are represented in FIG. 7.

Example 8 Use of the Anticalin FluA to Quench the Inherent Fluorescenceof Fluorescein

Complex formation was followed by fluorescence titration of a solutionof fluorescein with different concentrations of the anticalin FluA.Here, the decrease in the intensity of the intrinsic fluorescence of thefluorescein ligand was measured. The measurements were made with a LS 50B fluorescence photometer (Perkin Elmer) at an excitation wavelength of490 nm (slit width 4 nm) and an emission wavelength of 512 nm (slitwidth 4 nm).

PBS supplemented with 1 mM EDTA of pH 7.4 (adjusted with NaOH) served asthe buffer system. All solutions used were sterile-filtered (0.45 μm).The solution of the purified anticalin FluA from the Example 3 wasdialyzed three times against this buffer. The concentration wasdetermined using an extinction coefficient of 59755 M⁻¹ cm⁻¹ for FluA.

For a measurement series, a set of 15 solutions with a constantfluorescein concentration of 1 μM and varying respective proteinconcentrations from 0 to 10 μM was made, each in a total volume of 120μl. For this purpose, each 6 μl of a 20 μM solution of fluorescein inthe mentioned buffer were mixed with different volumes of the anticalinstock solution and brought to a total volume of 120 μl with buffer.

For the measurement of the fluorescence intensity as depending on therespective concentration of the anticalin, the individual solutions weretransferred to a quartz microcuvette and thermostatted in the sampleholder of the photometer for 1 minute at 25° C. The fluorescence signalwas subsequently measured as an average value over 10 seconds. Aftersubtraction of the fluorescence value for the buffer, the signals werescaled to an initial value of 100%. A further correction of the measuredvalues proved to be unnecessary.

It appeared that the initially high fluorescence intensity of thefluorescein decreased significantly with increasing concentration of theanticalin until only a very small fluorescence could be measured. Themeasured values obtained for the titration series were fitted accordingto the following formula by non-linear regression with the help of thecomputer program Kaleidagraph (Abelbeck Software):$F = {{\left( {\lbrack L\rbrack_{t} - \lbrack P\rbrack_{t} - K_{d}} \right)\frac{f_{L}}{2}} + {\left( {\lbrack P\rbrack_{t} + \lbrack L\rbrack_{t} + K_{d}} \right)\frac{f_{PL}}{2}} + {\left( {f_{L} - f_{PL}} \right)\sqrt{\frac{\left( {\lbrack P\rbrack_{t} + \lbrack L\rbrack_{t} + K_{d}} \right)^{2}}{4} - {\lbrack P\rbrack_{t}\lbrack L\rbrack}_{t}}}}$

Here, F is the scaled fluorescence intensity and [L]_(t) theconcentration of fluorescein (1 μM). [P]_(t) represents the totalconcentration of FluA in the respective titration step. f_(PL) and K_(d)were fitted to the measured data as free parameters and they denote thefluorescence coefficient of the complex of the anticalin FluA withfluorescein as well as their thermodynamic dissociation constants.

The measured fluorescence intensity values measured and the fittedcurves are represented in FIG. 8. A value of 35.2±3.2 nM was determinedas the dissociation constant for the complex of the anticalin FluA andfluorescein. In the formation of the complex with the anticalin FluA,the fluorescence intensity of fluorescein was quenched by 99.7±0.3%,i.e. it was almost quantitatively quenched. In the corresponding controlexperiment with the recombinant BBP, no comparable fluorescencequenching was found.

1-10. (canceled)
 11. The lipocalin according to claim 21, wherein saidloops have been mutated at one or more positions selected from the groupconsisting of amino acids 34, 35, 36, 37, 58, 60, 69, 88, 90, 93, 95,97, 114, 116, 125 and 127 of said bilin-binding protein, wherein saidamino acid position number is based on SEQ ID NO:
 20. 12. The lipocalinaccording to claim 20, wherein the ligand is a chemical compound in afree form.
 13. The lipocalin according to claim 20, wherein the ligandis a chemical compound in a conjugated form.
 14. The lipocalin accordingto claim 20, wherein the ligand is a peptide.
 15. The lipocalinaccording to claim 14, wherein the ligand is conjugated to anothermolecule.
 16. The lipocalin according to claim 20, wherein the lipocalinmutein is bound to a solid phase, and wherein the bound lipocalin muteinbinds a ligand.
 17. The lipocalin according to claim 20, wherein thelipocalin comprises a fusion protein and is bound to a solid phase, andwherein the bound lipocalin binds a ligand.
 18. The lipocalin accordingto claim 20, wherein the ligand is coupled to another molecule.
 19. Thelipocalin according to claim 20, wherein the lipocalin is coupled to acompound selected from the group consisting of an enzyme, an antibody, aradioactive moiety and a molecule with a defined binding characteristic,wherein the compound is detectable in a detection reaction.
 20. Anisolated lipocalin having a cylindrical β-pleated sheet supersecondarystructural region comprising eight β-strands connected pair-wise by fourloops at one end to define thereby a binding pocket, said four loopscorresponding to those segments which comprise amino acids 28-45, 58-69,86-99, and 114-129 of the linear polypeptide sequence of the bilinbinding protein of Pieris brassicae SEQ ID NO: 20, wherein at least oneamino acid of each of at least three of said four loops has been mutatedand wherein said lipocalin is effective to bind a ligand with adeterminable affinity of at least 10⁵ M⁻¹.
 21. The lipocalin of claim20, wherein at least one amino acid of each of said four loops has beenmutated.
 22. The lipocalin of claim 20, wherein the lipocalin is chosenfrom the group consisting of the bilin-binding protein of Pierisbrassicae, human retinol-binding protein, and human apolipoprotein D.23. The lipocalin of claim 22, wherein the mutated amino acids in theregion of the four peptide loops represent the sequence positions 34-37,58, 60, 69, 88, 90, 93, 95, 97, 114, 116, 125 and 127 of bilin-bindingprotein SEQ ID NO: 20, the sequence positions 34-37, 59, 61, 70, 87, 89,92, 94, 96, 113, 115, 123 and 125 of apolipoprotein D SEQ ID NO: 19, orthe sequence positions 32-35, 57, 59, 73, 90, 92, 95, 100, 102, 119,121, 131, and 133 of human retinol binding protein SEQ ID NO:
 18. 24.The lipocalin of claim 20, wherein the ligand is a chemical compound.25. The lipocalin of claim 14, wherein the peptide is a polypeptide. 26.The lipocalin of claim 20, wherein the ligand is a hapten.